In metastatic breast cancer, the Epidermal Growth Factor Receptor family of transmembrane tyrosine kinases (ErbB) drives proliferation and metastasis. EGFR, HER2 and ErbB3 are known to be highly expressed and active in both the HER2+ and Triple Negative/Basal subtypes of breast cancer, and have been targeted with varying effectiveness. Importantly, targeting of a single receptor member typically results in activity in the remaining receptor members, making simultaneous targeting of all receptors a needed therapeutic development. Furthermore, while the tyrosine kinase domain of these receptors is a significant driver of transformation, kinase inhibitors fail to induce cell death, indicating that a non-kinase function of the ErbB receptors is important in driving cancer progression. While the tyrosine kinase activity of these receptors is well known, less appreciated are the non-canonical activities of this family, including the modulation of mitochondrial function calcium signaling and nuclear translocation and activity as transcriptional co-factors. These functions are regulated simultaneously by the juxtamembrane domains (JD) of the ErbB receptors, which are also responsible for active homo- and hetero-dimerization. The non-kinase JD forms anti-parallel dimers between the receptors upon ligand binding, resulting in receptor transphosphorylation, Calmodulin activation, intracellular trafficking of the receptors and signal transduction. These functions drive ErbB-dependent survival, migration, growth and therapeutic resistance. We have previously demonstrated that peptides mimicking this JD can act in a dominant-negative fashion, promoting the formation of non-functional ErbB dimers (consisting of EGFR, HER2 and ErbB3) that induce rapid, ErbB-dependent cell death. Using Cell Penetrating Peptides synthesized in tandem with the JD (EJ1), peptides rapidly cross the plasma membrane, bind the ErbB receptors, and induce cell death in cell lines and PDX lines grown in culture. Death results from a combination of apoptotic and necrotic mechanisms, due to inactivation of ErbB kinases, and modulation of mitochondrial and calcium signaling. Initial results showed that, although these peptides display rapid ErbB-dependent cell death in vitro, the peptides have moderate efficacy in vivo. To stabilize in vivo activity, the active peptide (which is an alpha-helix) was stabilized with hydrocarbon staples (SAH- EJ1), resulting in a 10-fold increase in the activity of the peptide. Based on these data, we hypothesize that SAH-EJ1 will serve as an effective and stable therapeutic for the treatment of ErbB- dependent breast cancer. We plan to investigate this hypothesis by 1) Performing the preliminary in vivo Pharmacokinetics studies for SAH-EJ1, and 2) Determining the activity of SAH-EJ1 in PDX models of metastatic breast cancer that express EGFR, HER2, and/or ErbB3.